1. Field of the Invention
Exemplary aspects of the present invention generally relate to a cooling device for an image forming apparatus such as a printer, a facsimile machine, and a copier, and an image forming apparatus including the cooling device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile capabilities, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
Although differing depending on types of toner and types and speed of conveyance of the sheet, the fixing device is generally controlled to have a temperature of about 180 C.° to 200 C.° so as to instantly melt toner and fix the toner image onto the sheet. Therefore, the temperature of the sheet immediately after passing through the fixing device is high, typically about 100 C.° to 130 C.° depending on the thermal capacity of each sheet such as specific heat and density. Because the melting point of toner is lower than the temperature of the sheet heated by the fixing device, the toner on the sheet is still slightly soft immediately after the sheet has passed through the fixing device, and remains adhesive until the sheet is sufficiently cooled. Consequently, in a case in which multiple sheets discharged from the fixing device are sequentially stacked one atop the other on a discharge tray during continuous image formation, such soft toner on one sheet may adhere to the next sheet, resulting in blocking and considerable image degradation.
In addition, when multiple sheets that are still warm are sequentially stacked one atop the other on the discharge tray after being discharged from the fixing device, the heat retained by the stacked sheets softens the toner on the sheets and the weight of the stacked sheets compresses the sheet and possibly causing them to stick together. If stuck sheets are forcibly separated, the toner images formed on the sheets may be damaged or destroyed. For these reasons, the sheets after the fixing process need to be sufficiently cooled.
There is known a cooling device including a single cooling member that contacts an inner circumference of an endless conveyance belt that conveys the sheet. The cooling member absorbs heat via the conveyance belt from the sheet conveyed by the conveyance belt to cool the sheet discharged from the fixing device. The sheet heated by the fixing device is cooled by the cooling member while being conveyed by the conveyance belt. Therefore, the temperature of the sheet is lowered as the sheet approaches a downstream portion of the cooling member in a direction of conveyance of the sheet.
With such a configuration, the amount of heat absorbed by the cooling member is also decreased toward the downstream portion of the cooling member. Therefore, an upstream portion of the cooling member is hotter than a downstream portion thereof. However, because a single cooling member is used to cool the sheet from upstream to downstream in the direction of conveyance of the sheet, heat from the hotter upstream portion of the cooling member is transmitted to the downstream portion. Consequently, the downstream end of the cooling member cannot be kept low, thereby degrading cooling efficiency and possibly preventing sufficient cooling of the sheet.
In another approach, an image forming apparatus includes a cooling device having a block-type cooling member provided downstream from the fixing device in the direction of conveyance of the sheet. A channel through which liquid coolant flows from downstream to upstream is formed inside the cooling member, and the cooling member contacts the sheet to cool the sheet while the sheet is conveyed past the cooling device. Thus, the sheet discharged from the fixing device is cooled by the cooling member included in the cooling device. Accordingly, toner on the sheet is also cooled and cured, thereby preventing blocking. The liquid coolant enters the cooling member from an inlet provided at a downstream end of the cooling member and flows through the channel to an outlet provided at an upstream end of the cooling member. Accordingly, the cooling member heated by heat absorbed from the sheet is cooled by the liquid coolant.
In a case in which the liquid coolant flows through the cooling member from upstream to downstream so as to cool the sheet, upstream and downstream portions of the cooling member sequentially absorb heat from the sheet. Consequently, the temperature of the liquid coolant flowing through the cooling member increases toward the downstream portion of the cooling member. As a result, a difference in temperature between the sheet and the liquid coolant flowing through the downstream portion of the cooling member also decreases, thereby degrading cooling efficiency.
By contrast, when the liquid coolant flows through the cooling member from downstream to upstream as described in the above example, the sheet can be cooled by the cooler liquid coolant at the downstream portion of the cooling member compared to the case in which the liquid coolant flows through the cooling member from upstream to downstream. As a result, the difference in temperature between the sheet and the liquid coolant flowing through the downstream portion of the cooling member can be increased, thereby efficiently cooling the sheet at the downstream portion of the cooling member.
However, again, because heat absorbed from the sheet by the upstream portion of the cooling member is transmitted to the downstream portion, the temperature of the liquid coolant flowing through the downstream portion of the cooling member is increased. Therefore, even in a configuration in which the liquid coolant flows through the cooling member from downstream to upstream, thermal transmission within the cooling member increases the temperature of the liquid coolant flowing through the downstream portion of the cooling member, thereby degrading cooling efficiency at the downstream portion of the cooling member.